Mechanism of ring cleavage of acetylcyclopropanes by metal–ammonia solutions

Abstract

The reduction of acetylcyclopropanes by metal–ammonia solutions involves cleavage of the cyclopropane ring. A mechanism for this process is proposed which accounts for the observations that (i) substituent effects in the cleavage to form the saturated ketone indicate rearrangement of a carbanionic species, (ii) the overall reduction to saturated ketone involves two electrons per molecule, (iii) reductive dimerisation characteristic of a radical species occurs when low concentrations of lithium are employed, and (iv) some starting ketone is always recovered unchanged. The ratio of the two cleavage products formed from 1-acetyl-2,2-dimethylcyclopropane was observed to vary linearly with the lithium concentration. Two possible explanations of this effect are discussed. The preferred direction of cleavage of 2-substituted cyclopropylcarbinyl radicals has been studied. The Nickon–Sinz reaction of N-(trans-2-methylcyclopropylmethyl)toluene-p-sulphonamidegave products which indicate that decomposition of the intermediate alkyldi-imide follows a radical rather than a carbanionic pathway.